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Patrick Guilfoile: When sugar tastes bitter, or how cockroaches avoid poison

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BEMIDJI -- My wife is not a fan of cockroaches.

In fact, the words loath or disgust most closely describe her reaction to these six-legged scavengers. Even the fact our youngest child's first word was "ant" as he pointed at a cockroach crawling under a kitchen counter in our apartment in Boston, didn't endear the little creatures to her heart.

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My wife is in good company. Most people do whatever they can to eradicate cockroaches from their homes. One common approach for killing cockroaches is to use baits containing poison. These baits are often sweetened with glucose to induce the cockroach to swallow the insecticide. The good news is that these baits often work very well. The bad news is that cockroaches are evolving an aversion to the sweet-tasting baits, making it even harder to control them.

In general, animals, including humans, use our taste buds to make a snap judgment about what we eat. As a rule, if something tastes bitter, it is more likely to be harmful. On the other hand, if something tastes sweet, it normally is not dangerous. In the case of the bait-averse cockroaches, they recognize sweet as bad, even potentially deadly.

In the interest of better understanding how cockroaches learned to avoid sweetened, poisoned baits, researchers from North Carolina State University studied cockroach taste buds. They compared bait-averse cockroaches with those cockroaches that still eat glucose-sweetened foods (and baits). The researchers measured the electrical activity of different taste buds on the cockroaches' taste organ. When the appropriate type of food sticks to a taste bud, it gives off an electrical signal. One type of taste bud detects sweet foods; the other type detects bitter tastes, like caffeine.

In normal cockroaches, glucose stimulates sugar taste buds and that encourages the cockroach to consume the sugar-containing material whether food or poison. In the bait-averse cockroaches, glucose stimulates both the sugar taste bud and the bitter taste bud. The signal from the bitter taste bud overwhelms the signal from the sugar taste bud. Consequently, the stimulated bitter taste buds send a signal to the cockroach's nervous system to avoid the bait.

This trait of avoiding glucose-containing baits appears to be widespread. Researchers collected 19 different cockroach populations, and found seven of the 19 were glucose-averse (36 percent). This suggests a strong selection for the glucose-avoidance trait, and the rapid evolution of the trait in the 30 years since these baits first came available. It highlights the power of natural selection to allow pests, such as cockroaches, to survive, often in unexpected ways, an onslaught of pesticides and other potentially deadly treatments. It is no wonder cockroaches have survived for tens of millions of years. But, with our ability to understand in detail how cockroaches evolve, we may still have an opportunity to get the upper hand over these insect pests.

More information is available in the article "Changes in taste neurons support the emergence of an adaptive behavior in cockroaches" by Ayako Wada-Katsumata, Jules Silverman and Coby Schal in Science 340: 972-975, May 24, 2013.

PATRICK GUILFOILE has a Ph.D. in bacteriology and is the associate vice president at Bemidji State University.

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